It is known that diamond has a negative electron affinity. It is also known that diamonds emit electrons because of this negative electron affinity and, indeed, emit at much lower fields than other common electron emitters such as molybdenum or tungsten. This is currently not a controllable function. The emitter current is often much lower than would be predicted and some samples that seem to have all the criteria for emission often do not emit at all.
Because of the large energy bandgap (5.5 eV) between the valence and conduction bands, the number of carriers in a diamond semiconductor is necessarily low at room temperatures. Currently known dopants have very large ionization energies in diamond (on the order of 1 eV) and hence contribute poorly to conduction below +250.degree. C. So even though the effective work function of diamond is positive and considered to be somewhere between 0.2 eV and 0.7 eV (even though its electron affinity is negative) its saturation current is low. Raising the saturation current is the primary problem to be solved.